The present invention relates to a process for producing plate-like barium ferrite particles for magnetic recording, and in more detail, relates to a process for producing plate-like barium ferrite particles for magnetic recording, represented by the formula: EQU BaFe.sub.12-x-y M.sub.x Ti.sub.y O.sub.19
wherein x and y are independently not more than 1.2 and M is Co(II) or Co(II) and a divalent metal other than Co, which comprises adding a compound of Si(IV) to an aqueous highly alkaline suspension of ferric hydroxide containing Ti(IV), Ba(II) and Co(II) or Co(II) and a divalent metal other than Co as M(II), is such an amount that the atomic ratio of Si(IV) to Fe(III) is in the range of 0.0025 to 0.036, and autoclaving the resultant aqueous highly alkaline suspension of ferric hydroxide at the temperature of 200.degree. to 300.degree. C., thereby obtaining the plate-like barium ferrite particles having the thickness thereof controlled according to the atomic ratio of Si(IV) to Fe(III).
In recent years, accompanying with the propagation of VTR audio component, word processor and computor, non-acicular ferromagnetic particles of a favorable dispersibility provided with a suitable coercive force (Hc) and a large magnetization (M) have been required as the magnetic material for recording, particularly as the magnetic material for the perpendicular magnetic recording.
In general, as a method for producing barium ferrite particles, a method of treating an aqueous alkaline suspension containing Ba(II) and Fe(III) in an autoclave as a reactor (the method is hereinafter referred to as "the autoclaving method") has been known, and by selecting the reaction conditions in the autoclaving method, barium ferrite particles precipitate.
The thus precipitated particles are hexagonal plate-like particles, and the distribution of the particle size and the average size of the particles vary according to the reaction conditions, and in result, the particles have a different magnetic properties.
In addition, one of the factors which contribute the favorable dispersibility of the particles is that a width/thickness ratio (the ratio of the average diameter of the c-plane of the crystal of the particle (width of the particle) to the thickness of c-axial direction of the crystal thereof) is in a certain specified range, and for producing the particle of a favorable dispersibility, it is demanded to offer a method for freely controllig the thickness of the particle corresponding to the average diameter (width) of the produced plate-like particles.
As a result of the present inventor's studies for finding a method of freely controlling the thickness of the plate-like barium ferrite particle formed by the autoclaving method so as to obtain plate-like barium ferrite particles which are favorable as magnetic powdery particles for use in magnetic recording, the present inventors have found the followings.
Namely, in the process for producing plate-like barium ferrite particles by autoclaving an aqueous highly alkaline suspension of ferric hydroxide containing Ba(II), Ti(IV) and at least Co(II) as M(II) at a temperature in a range of from 200.degree. to 330.degree. C., the thickness of the thus formed plate-like barium ferrite particles of BaFe.sub.12-x-y M.sub.x Ti.sub.y O.sub.19 wherein x, y and M are as defined above, can be freely controlled by preliminarily adding a compound of Si(IV) to the aqueous alkaline suspension of ferric hydroxide, in such an amount that the atomic ratio of Si(IV) to Fe(III) is in a range of 0.0025 to 0.036.
The present inventors have attained the present invention based on the above-mentioned finding.